1. Field of the Invention
This invention relates to a thermal recording apparatus using a thermal head, and more particularly to a thermal recording apparatus which makes it possible to record even on a rough-surfaced paper sheet or to perform half-tone recording.
2. Description of the Related Art
Since it is simple in structure, high in reliability and convenient to maintain, thermal recording and thermal transfer recording are currently the mainstream in facsimile and color printing, respectively.
FIG. 13 of the accompanying drawings shows a typical conventional thermal transfer recording apparatus. This thermal transfer recording apparatus comprises a thermal head 1 having heat generating elements (not shown) for thermal transfer to a recording paper 6, a platen roller 21 against which the thermal head 1 is to be pressed, and an ink sheet 20 having an ink layer in which a plurality of sets of color regions of yellow (Y), magenta (M) and cyan (C), or black (K) if necessary, are arranged in a row. The ink sheet 20 also has a thin base film such as of polyethylene terephthalate (hereinafter abbreviated as "PET") formed over the ink layer. Generally the recording paper 6 is a high-quality paper or a dedicated paper having a high degree of smoothness.
In color recording, firstly the yellow (Y) region of the ink sheet 20 is conveyed into the gap between the thermal head 1 and the platen roller 21 with the recording paper 6. During recording, the heat generating elements of the thermal head 1 are energized for a predetermined time to generate joule heat and then the heat is transferred to the ink sheet 20. As a result, according to a record signal, solid ink on the ink sheet 20 is softened (liquefied) and is then transferred in part to the recording paper 6. The same operation is repeated in order for every color. In monochromatic recording, the same recording is made on the ink sheet 20 whose ink layer consists of only a black (K) region.
The common problem with the conventional thermal transfer recording apparatuses has been only a limited printing speed and the need for smooth-surfaced paper. For a better printing speed, on the one hand, it is now customary to improve the thermal head itself or to control the temperature of the heat generating elements precisely.
On the other hand, studies are currently being made to enable recording on plain paper sheets (PPC paper), which are widely used in copying machines. However, in thermal transfer recording, the process to transfer ink to a paper depends on the roughness of the paper surface; for example, when a rough paper is used, mistransfer such as voids would occur to deteriorate the quality of recording. This results because ink could not adhere to recesses or dents on the paper surface but could be transferred to only other surface portions.
In a serial printer, a fairly good printing can be obtained on a low-smoothness paper such as PPC paper by transferring ink in a bridge form; whereas in a line printer, an adequate result can not be obtained. For example, a publication "IMAGING Part 2" (Electronic Photographic Society's Hard Copy Series edited by the Electronic Photographic Society, pages 65-73) issued by Photographic Industries Publisher Co. Ltd. (a Japanese corporation) discloses: the concept of optimizing the break elongation rate and the viscoelasticity of ink at the ink peeling temperature (improvement to the ink sheet); an edge-type head with which the ink peeling timing is reduced in order to transfer ink while the ink viscosity is low at a high temperature and in which an adequately large angle of peeling ink can be realized (improvement to the ink peeling method); and the concept of bulging the graded layer right under the heat generating elements to cause an intimate contact with the recording paper (improvement to the head structure). In addition, studies are currently being made to apply an increased amount of power to cause ink to flow into recesses on the paper surface.
When recorded on a low-smoothness paper such as bond paper, voids may occur in parts to varying degrees. When applying an increased amount of power to eliminate any voids, ink would bolt on the paper.
The reasons why the edge peeling method could not eliminate the foregoing problems are that the solidifying time of ink was usually several ms and that solidification terminated before recording reached the peeling position in a line printer. Specifically, with the thermal head of 12 dots/mm in which the heat generating elements are located in a position 1 mm from the edge, recording of 1 line must be completed within 5/12 ms. Since the number of heat generating elements per line is about 2500, it requires a power supply of over several kW to meet the foregoing conditions, which is not practical.
Another problem with thermal transfer recording is that half-tone recording could not be achieved; that is, only binary recording, i.e. recorded or not recorded, was possible. Generally thermal transfer recording done using energy immediately after the recording density has become saturated in the relationship between energy and recording density. This is because if transient energy before the recording density becomes saturated was used, the recording density would have varied widely to deteriorate the image quality considerably. This is partly because the probability of whether or not ink is transferred to the paper would be 50% and partly because components constituting as image noise would increase in this operating region. Therefore in half-tone recording, it was inevitable to use pseudo gradation such as area gradation so that resolution becomes impaired.